1. Field of the Invention
This invention relates to apparatus for removing particulate deposits that collect on the inner wall of a conduit that conveys wet and dirty gas and the like, and more particularly to a blowback device for developing a fluid counterflow in a conduit that conveys gaseous products of combustion from a combustion process to a gas analyzer for the purpose of cleaning the inner wall of the conduit and/or preventing an explosive mixture of combustible material from entering the gas analyzer.
2. Description of the Prior Art
A gas analyzer of the type disclosed in U.S. Patent application Ser. No. 739,712, entitled "Method And Apparatus For Conveying A Gas Sample Through An Analyzer Chamber" filed Nov. 8, 1976, includes a pair of gas sample flow loops that are connected to each other and are operable to convey a sample of combustion product gas from a combustion process to a gas analyzer for the purpose of measuring the combustibles and oxygen concentration in the gaseous sample. An eductive loop has an inlet portion that collects a sample of the combustion product gas from the combustion process and conveys the sample to an outlet. With this arrangement, a sample of the combustion product gas is withdrawn from the combustion process for analysis. A convective loop is connected in fluid communication with the eductive loop at a point upstream from an aspirator, which is located adjacent to the outlet of the eductive loop and is operable to maintain circulation of the gas sample through the eductive and convective loops. A gas sensor of the type manufactured and sold by Thermox Instruments, Inc. of Pittsburgh, Pa., is positioned in the convective loop and is operable to analyze a sample of the gas taken from the eductive loop to make a determination of net excess oxygen or net excess combustibles in the gas sample.
When a dirty gas having particulate matter entrained therein is sampled, the particulate matter builds up after a period of time on the inner walls of the loop conduits. If this condition is allowed to continue in the sampling of a dirty gas, the conduits will eventually become clogged limiting or terminating flow entirely to the gas analyzer. Consequently, the gas sensor ceases to function at full efficiency. Furthermore, the presence of an explosive mixture of combustible material in the conduits of the gas analyzer presents a hazard of an explosion occurring in the gas analyzer which operates at elevated temperatures. This condition could occur, for example, in the event the flame of a combustion process fails, thereby permitting an explosive mixture of gas and air to enter the conduits of the gas analyzer.
To avert plugging of the conduits by the buildup of particulate matter on the conduit walls and to clean the conduits, blowback devices have been utilized as disclosed in the above mentioned patent application to develop a flow of air through the eductive and convective loops to dislodge particulate matter from the inner walls of the loop conduits and flush the matter from the conduits. It is the conventional practice in the backblowing of a fluid conveying conduit to utilize a conventional rotary valve which when rotated in a preselected direction discontinues flow of the gas sample through the eductive loop and permits air under pressure to enter the eductive loop and flow in a direction opposite to the flow of the gas sample. In this manner, the eductive loop is flushed of built-up deposits and unrestricted flow of the gas sample to the gas analyzer is maintained.
Conventional valves utilized in backblowing contain seals, which after prolonged use of the valve are subject to leakage and malfunction when exposed to a gas sample at an elevated temperature. In addition, mechanical valves must be positioned in a location which accommodates the size of the valve and permits access for actuation. For large valves this presents a problem because the loop conduit must be structured to provide access to the valve. Frequently, the limitations placed on the location of the blowback valve interferes with the operation of the gas analyzer. Also, during normal operation of the gas analyzer when the gas sample flows through the loop conduits, mechanical blowback valves have a tendency to restrict the gas flow through the loop conduits.
There is need for an apparatus to develop a counterflow in a conduit conveying a fluid with particulate matter entrained therein to remove deposits of the particulate matter from the inner wall of the conduit. The counterflow apparatus must be easily adapted to a preselected conduit structure associated with a gas analyzer. The counterflow apparatus must further provide unrestrictive flow of a gas sample through the conduit when counter flow is not required and be operable to prevent leakage of the gas sample during counterflow operations and withstand the elevated temperatures of the gas sample.